A stabilizer that is used for vehicles such as automobiles is a device for maintaining roll rigidity of the vehicles. FIG. 1 is a perspective view showing a construction of a stabilizer that is connected to a suspension device of a car. The stabilizer 10 includes a torsion part 11, an arm part 12, and a shoulder part 13, for example, and is formed approximately in the shape of the letter U. In the stabilizer 10, the torsion part 11 is attached to a car body (not shown) via bushes 3, and top portions of the arm part 12 are connected to the suspension devices 1 via stabilizer links 2. It should be noted that a tire (not shown) is attached to each of the axle parts 1A located at the left and right of the suspension devices 1.
In order to reduce the weight of the stabilizer, a hollow member such as a hollow pipe is used as a starting material. As the hollow member, a straight hollow member having constant outer diameter and inner diameter is generally used; however, in order to reduce weight further, a cross-section varying hollow member is suggested, which includes a portion at which diameter of the material varies along the central axis (for example, see Japanese Utility Model Publication No. Sho58 (1983)-45130, Japanese Examined Patent Publication No. Sho62 (1987)-21642, Japanese Patent No. 3350446.
In the above three documents, since the member includes a portion at which the diameter of the material varies along the central axis, it is difficult to produce and the production cost thereof is high. However, the Applicant of the present invention suggested limiting the cross sectional area of the material to two types to solve the problem (see Japanese Unexamined Patent Application Publication No. 2011-168101).
In the production of the stabilizer that includes the cross-section varying hollow member, the torsion part, arm part, and shoulder part are formed by performing a bending processing of the cross-section varying hollow member for example so as to obtain the cross-section varying hollow member having the shape of the letter U, and then a heating treatment such as quenching or the like of the cross-section varying hollow member is performed. In this case, as the heating method to perform the heat treatment, an energization heating is generally performed. In the energization heating method, an electrode is arranged at each of the two ends of the cross-section varying hollow member having the shape of the letter U, and electric powder is applied between the electrodes so as to heat the entirety of the cross section varying hollow material.
However, since the cross-section varying hollow member has a portion at which cross sectional area varies along the central axis, temperature increase rate greatly differs between a portion having a large cross sectional area (large cross sectional area part) and a portion having a small cross sectional area (small cross sectional area part) in the above method by the energization heating. Therefore, before the temperature of the large cross sectional area part reaches a predetermined temperature for example, the temperature of the small cross sectional area part greatly exceeds the predetermined temperature, and the small cross sectional area part may melt. As a result, it is difficult to heat the entirety of the cross sectional area varying hollow member uniformly until the temperature necessary to perform quenching.